JpaDialect implementation for Eclipse
Persistence Services (EclipseLink). Developed and tested against EclipseLink
1.0 as well as 2.0-2.3.

By default, this class acquires a EclipseLink transaction to get the JDBC Connection
early. This allows mixing JDBC and JPA/EclipseLink operations in the same transaction.
In some cases, this eager acquisition of a transaction/connection may impact
scalability. In that case, set the "lazyDatabaseTransaction" flag to true if you
do not require mixing JDBC and JPA operations in the same transaction. Otherwise,
use a LazyConnectionDataSourceProxy
to ensure that the cost of connection acquisition is near zero until code actually
needs a JDBC Connection.

This class is very analogous to TopLinkJpaDialect, since
EclipseLink is effectively the next generation of the TopLink product.
Thanks to Mike Keith for the original EclipseLink support prototype!

EclipseLinkJpaDialect

setLazyDatabaseTransaction

Set whether to lazily start a database transaction within an
EclipseLink transaction.

By default, database transactions are started early. This allows
for reusing the same JDBC Connection throughout an entire transaction,
including read operations, and also for exposing EclipseLink transactions
to JDBC access code (working on the same DataSource).

It is only recommended to switch this flag to "true" when no JDBC access
code is involved in any of the transactions, and when it is acceptable to
perform read operations outside of the transactional JDBC Connection.

This implementation invokes the standard JPA Transaction.begin
method. Throws an InvalidIsolationLevelException if a non-default isolation
level is set.

This implementation does not return any transaction data Object, since there
is no state to be kept for a standard JPA transaction. Hence, subclasses do not
have to care about the return value (null) of this implementation
and are free to return their own transaction data Object.